1. Field of the Invention
The present invention relates to an exposure apparatus used in image forming apparatus such as copiers, optical printers, and so on.
2. Related Background Art
In the image forming apparatus of this type, a photosensitive body is exposed to light by the exposure apparatus to form an image thereon in the form of a latent image. There are exposure methods including a laser beam method and a light emitting device method.
Since the laser beam method necessitates such optical components as a polygon mirror, lenses, etc., it has problems that compactification of apparatus is not easy and that it is hard to achieve quicker operation.
It is thus suggested to adopt the light emitting device method free of such problems. In this method, however, if an array of light emitting devices are constructed on a single substrate, the substrate will be expensive. Therefore, a plurality of cut chips were arranged in an array. In that case, there are challenges of decreasing level differences between the chips and having to keep gaps uniform.
In the light emitting device method the light from the exposure apparatus is radiated through an optical system such as a rod lens array or the like onto the photosensitive body. However, because the light from the exposure apparatus diffuses, efficiency of incidence of light to rod lenses becomes lower, so that the quantity of Light reaching the photosensitive body becomes smaller. There thus arose a problem that the light emitting devices had to be made to emit excessive light, in order to gain the sufficient quantity of exposure light.
Organic light emitting devices are sometimes used in the light emitting device method. However, emission intensities of such devices are distributed in a wide wavelength range and a half width of emission intensities against wavelength is thus wide, about 100 nm. Because of it, emission components at wavelengths off a wavelength region in which the photosensitive body has a peak of sensitivity, become not to act effectively to the exposure of the photosensitive body. Therefore, there also arises a problem that the exposure efficiency is lowered.
An object of the present invention is to provide an exposure apparatus overcoming the above problems.
An exposure apparatus according to the present invention has a light emitting device array formed on a single substrate, each device in the array comprising a micro-optical cavity, wherein the micro-optical cavity limits wavelengths of light from the light emitting device array, so as to make a wavelength at a peak of luminescence present in a half width region of sensitivity versus wavelength of a photosensitive body.
In order to accomplish the above object, an exposure apparatus of the present invention comprises an array of light emitting devices, each device comprising an anode, a cathode, an organic compound layer interposed between the anode and the cathode and adapted to emit light by a voltage placed between the anode and the cathode, and a semi-transparent reflecting layer,
wherein a micro-optical cavity is formed between the cathode and the semi-transparent reflecting layer and the cavity limits wavelengths of the emitted light, so that at least a peak of the emitted light is made present in a half width region of sensitivity versus wavelength of a photosensitive body.
In an embodiment of the present invention, the semi-transparent reflecting layer is in contact with the anode layer, the apparatus further comprises rod lenses for focusing the emitted light from the array of light emitting devices on the photosensitive body, or the apparatus further comprises microlenses for focusing the emitted light from the array of light emitting devices on the photosensitive body.
The present invention also provides an image forming apparatus comprising either of the above exposure apparatus; and the photosensitive body (211) for forming an image according to exposure by the exposure apparatus.